If the et is supersonic, then no signals can propagate back upstream. So if you have the correct nozzle exhaust, then I would think you should be good to go. Note that this only applies for a supersonic exit at the nozzle.

As long as the nozzle exit Mach number is greater than one then signals can't travel back upstream. If you have a converging-diverging nozzle then you'll have a hard time getting M=1 at the nozzle exit (nozzles don't work like that). So in that case you would need to model the nozzle itself. The only way to have a sonic condition at the exit is if you have a converging only nozzle. In that case, the sonic flow at the throat would be sufficient to ensure no signal propagation back upstream, as long as the throat remains choked.

What type of analysis are you doing? What type of information are you looking for. In general modeling the nozzle is a good idea. A velocity profile at a nozzle exit is not the idealized profile you are probably specifying.